A three member welded structure in which three members of a case, a cover, and a partitioning member are welded at the same time has been known (for example, refer to Patent Document 1). FIGS. 6 and 7 show an example in which the three member welded structure is applied to an automatic transmission oil filter. In the three member welded structure in which the three members are welded at the same time, a filter element 2 used as the partitioning member is fitted in a cover 1, and a case 3 is welded to the filter element 2 at the same time the case 3 is welded to the cover 1. In FIG. 7, hatched portions show a welding portion 4 between the case 3 and the cover 1 and a welding portion 5 between the case 3 and the filter element 2.
The automatic transmission oil filter is subjected to an airtightness test after welding. The airtightness test is carried out as follows: an opening 3a of the case is closed by a lid jig 6, and a negative pressure is produced at an opening 1a of the cover 1. In the state in which the negative pressure is produced, a change of pressure in the automatic transmission oil filter is detected.
Patent Document 1: Japanese Patent Laid-Open No. 11-277628